There were two Battles of Lostwithiel, both in the 1640s.

First Battle of Lostwithiel, 1642

Second Battle of Lostwithiel, 1644

After the overwhelming defeat at the Battle of Marston Moor, which cost him the entire north of England, Charles I continued to defy the Parliamentarian forces.

After defeating the Army of Sir William Waller at the Battle of Cropredy Bridge, The king marched west in pursuit of the Army of Robert Devereux, 3rd Earl of Essex, who was invading the Royalist stronghold of Cornwall. Charles's army vastly outnumbered that of The Earl of Essex, and had the advantage of surprise. On September 2 1644, Charles's army attacked the rear of Essex's force and quickly surrounded it at Lostwithiel in Cornwall, 30 miles west of Plymouth, attacking from both the north and the east at the same time. Lostwithiel was strategically important during the English Civil War since it held the key to Fowey, at the time a major seaport.

Most of the cavalry managed to escape, as did The Earl of Essex, but all of the infantry, about 6,000, and the artillery under Philip Skippon surrendered to the King on September 2nd. Charles wheeled about and marched toward London.

This setback for Parliament in Cornwall, and the last major victory for the Royalists, was reversed by Sir Thomas Fairfax leading the New Model Army at or near Tresillian Bridge, close to Truro on March 12 1645.

Battles of the English Civil Wars | History of Cornwall